The present invention relates to a system for hydraulically controlling an actuator such as an oil hydraulic cylinder having a piston.
The hydraulic cylinder is applied with pressurized oil from a pump driven by a motor, thereby causing the piston to be continuously and effectively shifted a distance dependent on quantity of oil supplied to the cylinder.
Japanese Patent Application Laid-Open 56-105102 discloses a hydraulic control system of an actuator for controlling the moving distance of the piston. Referring to FIG. 6, an actuator 1 has a piston 1a which reciprocates in a cylinder comprising two chambers defined by the piston 1a. Each of the chambers is communicated with a servo valve 12 through which oil is supplied. The servo valve 12 is controlled by an electronic control system having an servo amplifier 18. The servo amplifier 18 which is applied with an input signal el from an adder 9 is connected to a manually operated changeover switch 17 for selectively connecting the servo amplifier with a loop gain setting resistor 13 for controlling load exerted on the piston 1a of the actuator, or a loop gain setting resistor 15 for controlling moving distance of the piston 1a, in accordance with the position of the contact of the switch 17. The resistor 13 and 15 are connected to the servo valve 12 through amplifiers 14 and 16, respectively. By operating the switch 17, either the moving distance or the load of the actuator 1 is controlled.
The servo amplifier 18 is adjusted to nonlinearly produce an appropriate output signal e0 in accordance with the input signal e1 from the adder 9. The servo valve 12 is adapted to be operated to supply an appropriate quantity of oil to one of the chambers in accordance with a current i corresponding to the signal e0 applied from the amplifier 14 or 16.
The electronic control system is provided with a feedback control system comprising a moving distance detector 2 for sensing the actual moving distance of the piston 1a and a load detector 3 for sensing the actual load exerted on the piston 1a. The detector 2 is connected to an amplifier 7 through a variable resistor 6 and the detector 3 is connected to an amplifier 5 through a variable resistor 4. The amplifiers 5 and 7 are connected to the adder 9 through a changeover switch 8 which is operatively connected to the changeover switch 17 to be operated in synchronism therewith. An input signal representing a desired moving distance or a desired load applied to the adder 9 is compared with the signal from the detector 2 or 3. The difference between the signals is fed to the servo amplifier 18 to further control the piston 1a, thereby decreasing the deviation. Hence, the control system can accurately and stably control the actuator 1 with a servomechanism.
However, the servo valve is extremely expensive so that the manufacturing cost of the system is increased. In addition, the adder and the servo amplifier must be adjusted to obtain preferable operational characteristics thereof, the operation of which is complicated, so that the actuator cannot be accurately controlled.